An ideal thermal radiator, a so-called blackbody radiator, emits the maximum amount of energy physically possible at every wavelength λ. The spectral, i.e., wavelength-dependent, specific emission of such a blackbody radiator is described by Planck's radiation law. In thermal infrared radiation sources, the broadband radiant power emitted from a radiating area A is of interest, which is obtained by integration of Planck's radiation law over all wavelengths. The equationPs=σAT4,applies for this radiant power Ps, which is known as the Stefan-Boltzmann law of the black body radiator and in which σ denotes the Stefan-Boltzmann constant.
Real radiators are not blackbody radiators. The emitted radiant power thereof is less than that of the blackbody radiator of equal radiating area A and temperature T. This is because the real thermal radiator does not emit the maximum possible amount of energy at every wavelength λ. The ratio of real emitted amount of energy and maximum amount of energy which can be emitted is referred to as the emissivity E, which is in the range between zero and one. The emissivity of a blackbody radiator therefore has the value of one and is wavelength-independent. The emissivity of a real radiator, in contrast, is wavelength-dependent and is less than one.
The emitted radiant power of real radiators is furthermore reduced in comparison to blackbody radiators in that the radiating area A is not heated through homogeneously at the temperature T, since the heating element is generally fastened on a colder point, for example, on the housing, and this connection dissipates thermal energy from the heating element to the housing as a result of heat conduction. In addition, heat is dissipated via the surrounding gas. A temperature distribution T(A) thus forms on the area A, wherein areas having a maximum and a minimum temperature form on the radiating area. As a result, the emitted radiant power is therefore dependent on the mean temperature of the area A, which results from the arithmetic mean of the temperature distribution T(A).
The radiant power PrS of a real thermal radiator may therefore be described by an adapted Stefan-Boltzmann lawPrS=σεAT4 wherein ε represents the arithmetic mean over the wavelength-dependent emissivity ε(A)
      ɛ    _    =            1      λ        ⁢                  ∫        λ            ⁢                        ɛ          ⁡                      (            λ            )                          ⁢                                  ⁢        d        ⁢                                  ⁢        λ            and T represents the arithmetic mean of the temperature distribution T(A) on the radiating area A
      T    _    =            1      A        ⁢                  ∫        A            ⁢                        T          ⁡                      (            A            )                          ⁢        dA            
The radiant power is accordingly dependent on the fourth power of the mean temperature T and is directly proportional to the mean emissivity ε and the radiating area A. For a high radiant power, the radiating element accordingly has to have a high temperature and a high mean emissivity, which is as close as possible to one. In addition, a large radiating area A having homogeneous temperature distribution is necessary for a high radiant power. Many technical solutions exist for enhancing the emissivity, as described, for example, in the document DE 102012103662 B3.
All thermal radiators function according to the principle of Joule heating or also ohmic heating, i.e., when an electric current flows through a heating conductor, the electrical resistance of the heating conductor works against the current flow, whereby heat is generated. The heat thus resulting heats the heating conductor and is emitted from it via thermal radiation and heat conduction to the housing and/or to the surrounding gas. The heating conductor of an energy-efficient infrared radiator having high radiation yield emits a majority of the electrical energy generated by the applied voltage as thermal radiation again and therefore has to be designed so that the power loss as a result of the heat dissipation to the housing and/or to the surrounding gas is as small as possible.
The heat dissipation to the gas surrounding the radiating heating element or the radiating heating conductor, respectively, can be reduced by filling the housing of the infrared radiation source with an inert gas (for example, argon) and closing it gas-tight. Inert gases are distinguished by a substantially lower heat conductivity than that of air. The heat dissipation of a freestanding heating conductor to the housing of the infrared radiation source can be reduced by enhancing the heat resistance of the heating conductor. The heat resistance of a heating conductor is dependent on the material and its geometry. For typical heating conductor materials, for example, metals, it is proportional to the electrical resistance. A high electrical resistance is also to be considered very advantageous in circuitry, since, according to Ohm's law, lower currents flow in the case of an electrical voltage applied to the heating conductor than in the case of heating conductors having lower electrical resistance. It is described in Ott, T., et al: Efficient thermal infrared emitter with high radiant power, J. Sens. Sens Syst., 4, 313-319, doi:10.5194/jsss-4-313-2015, 2015 that an energy-efficient infrared radiation source has a freestanding heating conductor, which is ideally to be as long and thin as possible, to offer a high electrical resistance, a high heat resistance, and a large radiating area. Long freestanding heating conductors have the disadvantage, however, that they expand in an absolute manner more under thermal load than short ones. They are thus less mechanically stable than short heating conductors.
Thermal infrared radiation sources are primarily used in nondispersive infrared (NDIR) gas analysis. NDIR gas analysis is an optical method for determining the concentration of gases. The infrared radiation of the thermal emitter radiates through the cuvette having the fluid to be measured and is then incident on the sensitive area of the detector. To focus the highest possible share of the emitted infrared radiation of the radiation source onto the detector element, an additional optical unit is frequently integrated into the beam path. The radiating heating conductor therefore always has to be kept in the same position in relation to the optical unit at operating temperature, so that the focusing on the detector element is maintained. A further requirement for heating conductors is therefore mechanical stability. Typical heating conductor materials, for example, metals, expand under thermal load, which results in deformations in conjunction with the fastening thereof, for example, on the housing of the infrared radiator. The deformation is primarily dependent in this case on the temperature, the material used, and the heating conductor geometry.
Four different types of thermal infrared radiation sources have been used in applications up to this point in gas analysis: filament lamps, resistance coils, globars, and thin-film radiators.
Radiators having resistance coils and thin-film radiators are most frequently used in compact infrared-spectroscopy devices. In spite of the high emissivity thereof, globars are not suitable for use in compact infrared-spectroscopy devices, since they usually have to be operated with water cooling and may not be electrically modulated because of the large thermal mass thereof (DE 10 2012103 662 B3). Filament lamps, for example, incandescent lamps having tungsten coils, also do have a very high radiant power, since the temperature of the tungsten coils can be up to 3000° C. However, for this purpose they have to be operated in a protective gas atmosphere or in vacuum, for example, in a glass bulb. The glass is no longer sufficiently transparent for infrared radiation above 4.5 μm wavelength, however, so that this greatly restricts the field of use.
Radiators having resistance coils made of a thin, usually meandering structured metal heating conductor foil, for example, Kanthal or nickel-chromium (U.S. Pat. No. 5,939,726 A), display a broadband infrared spectrum. The radiating element is formed freestanding in this case and is fastened on several housing points, which hold the element in a fixed position and ensure the electrical contact. However, these radiators have the disadvantage that the radiating element has an excessively low electrical resistance because of its short length. Furthermore, the low heat resistance coupled to the low electrical resistance has the result that a majority of the electrical power is dissipated in the form of heat to the housing and is not emitted as desired thermal radiation. One advantage of this structure is the mechanical stability of the radiating element under temperature load, which results from the short heating conductor length. Furthermore, the radiation emitted on both sides can be used by a reflector integrated into the radiator housing.
The spiral heating conductors proposed in Ott, T., et al: Efficient thermal infrared emitter with high radiant power, J. Sens. Sens. Syst., 4, 313-319, doi:10.5194/jsss-4-313-2015, 2015 offer a sufficiently high electrical resistance and a homogeneous temperature distribution over the entire radiating area. The thickness thereof is in the range of several micrometers. These heating conductors are embodied as freestanding, so that the lower and upper sides of the radiating element can be used with a corresponding reflector installed into the housing. However, this heating conductor geometry has the disadvantage of the mechanical instability under thermal load resulting from the unsupported, i.e., freestanding long conductor length, which results in deformations of the radiating element under high temperatures.
In the case of thin-film radiators, for example, as known from DE 102004046705 A1, the radiator element is not formed freestanding, but rather is applied to a thin, nonconductive membrane. The lower side of the heating conductor layer thus cannot be used as a radiating area. Since the heating conductor metallization can be vapor-deposited in very thin form on the membrane, a high electrical resistance of the heating conductor results. In addition, the heating conductor is always held in one position and is thus mechanically dimensionally stable. Since the heat metallization and the nonconductive membrane consist of different materials, they expand differently under thermal load. The material which expands less strongly (generally the membrane), then obstructs the thermal expansion of the heat metallization. Since the radiators are generally operated pulsed, a compression of the heat metallization thus occurs cyclically, which results in cracks and decisively reduces the service life. The membrane radiators are thus limited in the operating temperature thereof, whereby they have a low radiant power. To produce the thin-film radiator, the radiating element consisting of a thin membrane and a heat metallization has to be fastened on a support frame, to be able to fasten it in the housing of the radiation source. This frame cannot be used as a radiating area and thus prevents the optimum utilization of the available installation space as a radiating area. A further disadvantage of thin-film radiators is the inhomogeneous throughheating (hotspot in the membrane center) of the heat metallization, since it is connected by the membrane directly to the heatsink (support frame), and heat is thus always dissipated.
Presently, there is no technical solution for an infrared radiation source having freestanding heating conductor, which operates energy efficiently due to a high electrical and thermal resistance and is distinguished by a high radiant power having long-term stability, which is ensured by a heating conductor, which only deforms slightly under thermal load and has a large radiating area having the most homogeneous possible temperature distribution.
It is therefore the object of the invention to specify a heating conductor geometry which avoids the above-mentioned disadvantages and may be integrated into compact infrared-spectroscopy devices.